I tried using Device and User Mode once and found that it was amazing. However, there were several bugs, where FreePBX directly addressed devices when it should have addressed the users (or perhaps the other way around). I reported that bug and was told that Device & User Mode was not supported. I’ve occasionally seen forum posts reporting issues with DUM as well, with the same response from the devs.
It is true that, behind the scenes, FreePBX always runs in Device & User Mode. However, when in “Extension” mode, the devices and the users get the same identifier (the extension number) and so the fact that FreePBX addresses a call to a device rather than the user or the other way around won’t matter since they are the same.
My assumption is that with the ability of psjip to support multiple devices registering to the same extension, there is no longer any need for an explicit Device & User Mode and so it has an even lower chance of any support going forward.